Small regulatory RNAs (sRNAs) of bacteria are short transcripts that serve regulatory functions by base-pairing to target mRNAs or interacting with regulatory proteins to affect gene expression. sRNAs have been widely studied in proteobacteria such as Escherichia coli, in which they control various aspects of metabolism, stress responses, and virulence. In firmacutes, there are relatively few examples of sRNA-mediated gene regulation and less is known about sRNA molecular functions and targets. In many proteobacteria, sRNA function and stability are dependent upon the RNA chaperone Hfq, while in firmacutes many sRNAs do not require Hfq for function and the impact of Hfq on physiology is poorly understood. Bacillus anthracis, the spore-forming firmacute that causes anthrax, produces two sRNAs, sRNA1 and sRNA2, encoded by the toxin plasmid pXO1. Expression of both sRNAs is positively regulated by AtxA, a key trans-acting regulator of virulence gene expression that is also encoded by pXO1. Our recent RNA-seq data reveal that sRNA2 affects expression of 51 genes. Among the most potentially significant targets are: (1) inhA1, encoding a secreted protease that facilitates crossing of the blood-brain-barrier by B. anthracis, (2) genes associated with branched-chain amino acid import and biosynthesis, and (3) genes predicted to encode chitinases and chitin-binding proteins. sRNA1 affects expression of only two genes, including a transcriptional regulator of unknown function. RNA-seq data from a double mutant ?sRNA1?sRNA2 show altered transcript levels for 118 genes, indicating synergy and functional overlap between the sRNAs. Notably, unlike most bacteria, B. anthracis has three Hfq genes; hfq1 and hfq2 on the chromosome and hfq3 on pXO1. AtxA positively regulates hfq2. Experiments proposed in this exploratory project will address our hypothesis that sRNA1 and sRNA2 are small regulatory RNAs that function with Hfq2 and/or Hfq3 to modulate virulence. In Specific Aim 1 we will explore the mechanism(s) by which sRNA1 and sRNA2 control gene expression. We will test for sRNA interactions with mRNAs of highly sRNA-regulated genes, and we will screen for other nucleic acids and proteins that interact with the sRNAs. We will compare the half- lives of the sRNAs in parent and hfq-null mutants. Finally, we will determine if the sRNAs affect the stability and/or translation of target mRNAs. In Specific Aim 2 we will determine if sRNA plays a role in B. anthracis virulence and pathogenesis using murine models for inhalational and late-stage anthrax. B. anthracis provides a distinct opportunity to investigate mechanisms of sRNA function because of the established relationship between sRNA expression and the master virulence regulator AtxA, and because the bacterium processes multiple Hfq paralogues. Information gained in these initial investigations will further knowledge of sRNA function in bacteria while enhancing understanding of B. anthracis pathogenesis. Our laboratory is uniquely poised to carry out the work because of our expertise in the physiology and genetics of this important Select Agent and model pathogen.